High cross correlation dynamic channel assignment for sectorized cells

Abstract: In a cellular communication system, an array of sector antennas is provided at each cell site for respectively covering sectors of the service area of the cell site. At each cell site, undesired signals of different channels are received by each antenna, and their magnitudes are determined. One of the signals is selected from each set of signals from the antennas depending on their magnitudes to produce channel signals respectively for the different channels. A list of candidate channels is determined corresponding to those of the channel signals whose qualities are higher than a threshold. One of the candidate channels is selected from the list depending on the magnitudes of those channel signals whose qualities are higher than the threshold. The selected candidate channel is assigned to a mobile station if the selected channel has acceptable quality. Otherwise, it is discarded from the list and the attempt is repeated to select a further candidate channel.

Description: The present invention relates generally to cellular communication systems, and more specifically to dynamic assignment of channels to mobile stations from a base station using an array of antennas respectively covering sectors of the service area of the base station. EP-A-0 430 173 describes a method of assigning radio communication channels to each of a plurality of mobile stations. The capacity of a cellular mobile communication system may be increased by reducing the size of the cells so that the total number of channels available per unit area is increased. This is achieved in practice by the process of "cell splitting" where new base stations are established. As the cells are split into smaller sizes, the co-channel interference from distant cells increases. One way of reducing the level of co-channel interference is to use directional antennas at base stations, with each antenna illuminating or covering a sector of the cell, whereas ominidirectional antennas are used in non-sectorized cells. Dynamic channel assignment schemes are known for efficient utilization of channels, and used for systems of non-sectorized cells. For sectorized cells, a dynamic channel assignment scheme is described in a paper "Channel Segregation, A Distributed Adaptive Channel Allocation Scheme for Mobile Communication Systems", Y. Furuya et al., Second Nordic Seminar on Digital Land Mobile Radio Communication, Stockholm, 1986, pages 311-315. According to the proposed scheme priorities are assigned to a list of available channels according to past records of co-channel interference. On receiving a request from a mobile station, a channel is selected from the list according to the priorities so that the mobile station has a reduced chance of encountering co-channel interference. A dynamic channel assignment scheme for efficient utilization of channels in sectorized cells is described in a paper "Adaptive Channel Allocation in a TIA IS-54 System", H. Andersson et al., Vehicular Technology Conference, 1992, pages 778 to 781. According to this scheme, carrier-to-interference ratio is used as criteria for assigning priorities to available channels. However, there is still a need for improvements in dynamic channel assignment scheme for sectorized cells in terms of the utilization of channels, the number of channel selection attempts for each request, and the probability of co-channel interference. More specifically, assume that, in a sectorized cellular communication network, first and second mobile stations are respectively communicating with first and second base stations. The signal from the first base station may be received by the distant, second mobile station as an undesired downlink signal and produce a significant level of interference, while the signal from the first mobile station may be received by the second base station as an undesired uplink signal. If the directional antenna of the second base station which is being used for the second mobile station is oriented in a direction away from the first base station, the undesired uplink signal from the first mobile station will produce a low level of interference. Since interference is currently detected only from a signal received by the antenna used for a mobile station of interest, the cross correlation for co-channel interference between the uplink and downlink signals of the mobile station is low. Because of the low cross correlation, the current dynamic channel assignment schemes are not satisfactory in respect of the utilization of channels, the number of channel selection attempts for each request, and the probability of co-channel interference. IEEE International Conference on Selected Topics in Wireless Communications, 1992, Vancouver, 25 June 1992, New York, US, pages 340-343, I. Brodie, "Performance of Dynamic Channel Assignment Techniques in a Cellular Environment" reports on the performance of several dynamic assignment techniques, and specific attention is given to sectorized configurations. This publication describes a base station having a plurality of antennas having different orientations to cover sector areas. A dynamic channel assignment technique is used to assign a channel according to carrier-to-interference ratio. The interference is calculated by summing the signal received from all the interfering sources transmitting on the same channel. It is described in this publication that, when a call attempt is made, the mobile station selects the base station and sector with the least path loss (not necessarily the closest) and, if there are any acceptable channels, one is selected using one of the DCA methods. It is an object of the present invention to provide dynamic assignment which ensures high degree of cross correlation for co-channel interference between uplink and downlink signals. This object is achieved with a dynamic channel assignment method according to independent claims 1, 3, or 4 The present invention will be described in further detail with reference to the accompanying drawings, in which: Fig. 1 is a block diagram of a cellular communication network in which the first embodiment of the present invention is implemented; Fig. 2 is a flowchart illustrating a sequence of operations performed by a base station and a mobile station according to the first embodiment; Fig. 3 is an illustration of directivity patterns of a four-antenna array used in the cellular network according to a second embodiment of the present invention; Fig. 4 is a flowchart illustrating a sequence of operations performed by a base station according to the second embodiment; Fig. 5 is a block diagram of a cellular communication network in which the third embodiment of the present invention is implemented ; Fig. 6 is a flowchart illustrating a sequence of operations performed by a base station according to the third embodiment; Fig. 7 is an illustration of directivity patterns of a three-antenna array used in the cellular network according to a fourth embodiment of the present invention; and Fig. 8 is a flowchart illustrating a sequence of operations performed by a base station according to the fourth embodiment of the present invention. In Fig. 1, there is shown an exemplary cellular communication network in which the first embodiment of the present invention is implemented. The network comprises a plurality of identical base stations arranged according to a repeat pattern based upon a known frequency reuse plan. Each base station has an array of four directional antennas "a", "b", "c" and "d" each with a directivity pattern that covers a sector of 90 degrees on the horizontal plane, thus dividing the coverage area of the base station equally into four sectorized areas A, B, C and D. For simplicity only four base stations 11 to 14 are illustrated, which may be adjacent to each other or spaced apart at least one cell-site distance. For purposes of control as well as speech communication, a downlink channel is established in a base-to-mobile direction and an uplink channel in a mobile-to-base direction. In this specification, a desired signal on any uplink and downlink channels is designated Dup and Ddn, respectively, and an undesired (co-channel interference) signal on any uplink and downlink channels as Uup and Udn, respectively. According to the first embodiment of the present invention, all base stations select any pair of uplink and downlink speech channels which are allocated to the cellular network as a whole. As a typical example, communications are established on a speech channel CH1 between mobile (or personal) station 22 and base station 12, on channels CH2, CH3 between mobiles 23, 24 and base station 13, and on channels CH4, CH5 between mobiles 25, 26 and base station 14. It is therefore likely that the antenna "b" of base station 11 will be exposed to the transmissions of undesired signals Uup from mobile stations 23, 24 and 25 (and hence the undesired signals are indicated as Uupb), and the antenna "c" of base station 11 will be exposed to the transmissions of undesired signals Uupc from mobile stations 22 and 26. In response to a call request signal from a mobile station 21, the base station 11 assigns a channel to the mobile station according to a channel assignment algorithm of the present invention, using undesired uplink signals from the surrounding base stations. The channel assignment algorithm of the first embodiment is shown in Fig. 2. In response to a call request signal sent from a mobile or personal station on an uplink control channel, the channel assignment routine begins. This call request is received by one of its antennas which is, for convenience, designated "x" (step 200). The base station proceeds to detect the power level of the received carrier and identifies it as a desired signal Dupx (step 201). A variable "i" is set equal to 1 at step 202 and a channel identified by the variable i is selected (step 203). Exit then is to step 204 to detect the power levels of undesired signals Uupa, Uupb, Uupc and Uupd, which are received by antennas a, b, c and d, respectively. Control proceeds to step 205 to determine the highest of the detected power levels and set it as Uupi as a possible interference signal. A ratio Dupx/Uupi is then calculated and compared with a predetermined value K (step 206). If the ratio is greater than K, control branches at step 206 to step 207 to set the channel i as a candidate channel, and proceeds to step 208 to determine whether all channels are tested. If the answer is negative, control branches at step 208 to step 209 to increment the variable i by one and returns to step 203 to repeat the process. If the decision at step 206 is negative, control branches out to step 208, skipping step 207. In Fig. 1, the base station 11 is assumed to receive the call request from mobile 21 at antenna "a" to detect the power level of the uplink signal Dupa, and to receive only one undesired signal for each channel. Therefore, with respect to channel CH1 (channel number i=1), the undesired uplink signal Uupc(1) from mobile station 22 is classified as having a highest power level at step 205. As the candidate determination process is repeated, channels CH2, CH3, CH4 and CH5 are successively tested, yielding ratios Dupa/Uup2, Dupa/Uup3, Dupa/Uup4, Dupa/Uup5. If the undesired uplink signals Uupc(1), Uupb(2) and Uupb(3) are relatively strong and their ratios Dupa/Uup1, Dupa/Uup2 and Dupa/Uup3 are determined at step 206 to be smaller than K, channels CH1, CH2 and CH3 will be discarded and the remaining channels CH4 and CH5 will be recorded in a memory as candidate channels having a high degree of cross-correlation with the desired downlink signal Ddn. Following the execution of step 208, control now enters a channel selection process. At step 210, a decision is made on whether candidate channels are present or not If not, control exits to step 217 to send a busy tone to mobile station 21. If the answer is affirmative, control branches at step 210 to step 211 to select a candidate channel corresponding to the signal Uupi having the highest power level from the list of candidates determined by step 207 and set the selected candidate channel as a channel j. Exit then is to step 212 to transmit a carrier on the selected channel j from base station 11 and send a command over the control channel to mobile station 21 to measure the power level of the transmitted carrier to determine a value Ddn. Base station 11 then ceases the transmission of the carrier and commands mobile station 21 to measure the level of noise detected on channel j to determine a value Udn. At step 213, mobile station 21 calculates the ratio Ddn/Udn and compares it with a prescribed threshold value M and sends a signal indicating whether the ratio is higher than the threshold. If the signal indicates that the ratio is smaller than the threshold (step 213), base station 11 removes the channel j from the list of the candidates (step 214) and returns to step 210 to check to see if candidate channels still exist. If the answer is affirmative, the channel of next highest is selected as channel j (step 211), and steps 212 and 213 are repeated. If the signal from mobile station 21 indicates that the ratio is greater than the threshold M, channel j is assigned to the mobile station (step 215) and a two-way connection is established on the assigned channel. The selection of a maximum power level at step 211 with priority enables base station 11 to eventually assign a group of channels having reduced frequency separations from each other at a cost of somewhat increase in co-channel interference, a compromise between the conflicting factors of channel utilization efficiency and co-channel interference. As a trade-off between these conflicting factors, other known channel assignment approaches are available. However, regardless of the approaches taken by the channel assignment subroutine (steps 210 to 216), the candidate selection subroutine (steps 203 to 208) of the present invention enables each base station of the cellular communication network to make an optimum decision on channel utilization efficiency and co-channel interference. Additionally, the present invention enables each base station to reduce the occurrence of co-channel interference and to reduce the number of repeated attempts to determine the best channel. According to a second embodiment of the present invention, the determination of the undesired signal of highest power Uupi at step 205 of Fig. 2 is modified. According to this modification, when an undesired signal is detected by two or more antennas, two of the signals having the highest and second highest power levels are selected and the highest power level is compensated for with a correction factor determined by the angle of arrival of the undesired signal. As illustrated in Fig. 3, each antenna has an identical directivity pattern P on a horizontal plane and the antenna directivity is represented by relative gain (a fraction of unity) with respect to the main axis of the antenna. If an undesired signal arrives at an angle α with respect to a reference angle (zero degrees) as shown in Fig. 3, it intersects the directivity patterns Pa and Pb of antennas "a" and "b" at points where relative gains are Ga and Gb, respectively, which are fractions of unity with respect to the main axes of their antennas. By representing the power levels of the signals received by antennas "a" and "b" as Uupa and Uupb, respectively, the power ratio Uupa/Uupb is equal to the antenna gain ratio Ga/Gb. Therefore, the relation Ga/Gb = Uupa/Uupb holds. The arrival angle of any undesired signal can be determined as being equal to α if the relation Ga(α)/Gb(α) = Uupa/Uupb is satisfied. With the arrival angle being determined for antenna a, the highest value Uupa is divided by relative gain Ga(α) to obtain Uupi which is equivalent to a signal arriving in the direction of the main axis of the antenna and detected with the gain of unity. In a practical aspect, the relative gain Gx(α) of antenna "x" is stored in a location of an antenna gain memory, not shown, in which relationships between arrival angles and relative antenna gains are mapped for each pair of adjacent antennas. Fig. 4 shows a flowchart for implementing the second embodiment of the present invention which is similar to the flowchart of Fig. 2 with the exception that step 205 of Fig. 2 is replaced with steps 400, 401, 402 and 403. Following the execution of step 204, control exits to step 400 to select an undesired signal of highest level from antenna x1 and set the power level of the signal as Uupx1. Exit then is to step 401 to select an undesired signal of the second highest level from antenna x2 and set the power level of the signal as Uupx2. At step 402, the antenna gain memory is searched for Gx1(α) and Gx2(α) which establish the relation Gx1(α)/Gx2(α) = Uupx1/Uupx2 and determines the angle of arrival α from this relationship. At step 403, Uupx1 is divided by Gx1(α) to determine the undesired signal of channel "i" with the highest power level Uupi. Step 403 is followed by step 206 to check to see if the ratio Dupx/Uupi is greater than the threshold value K. According to a third embodiment of the present invention which is shown in Fig. 5, each sector area of all the base stations is allocated a group of channels which are different from the channels of the other sectors of all the base stations but identical to those allocated to the sector areas of the other base stations which are illuminated by the antennas of the same orientation as the antenna of that sector area. For example, sector areas A of all cell site stations are allocated the same group of channels CH11, CH12, which differ from those allocated to the other sector areas. With this channel allocation plan, mobile station 21 of base station 11 is likely to be affected by undesired signals from antennas "a" of the other base stations 12, 13 and 14 as indicated by arrows Udn(11) on channel CH11 from base station 12, Udn(12) on channel CH12 from base station 13 and Udn(13) on channel CH13 from base station 14. According to this embodiment, cross correlation is detected between undesired signals received by a pair of antennas oriented in opposite directions. Thus, for mobile station 21, antennas "a" and "c" of base station 11 are used for determining the cross correlation. Fig. 6 shows a flowchart for implementing the third embodiment of the present invention which is similar to the flowchart of Fig. 2 with the exception that steps 204 and 205 of Fig. 2 are replaced with steps 600 and 601. Following the execution of step 203, control exits to step 600 to detect power levels of undesired signals Uupx and Uupy received at antennas "x" and "y" of mutually opposite orientations, using correlation receivers, not shown. Control advances to step 601 to determine the highest of the signals Uupx and Uupy and stores it as Uupi for channel i. Exit then is to step 206 to determine whether the ratio of the power level Dupx of downlink signal received at mobile station 21 to Uupi is greater than the threshold K. Therefore, in the example illustrated in Fig. 5, signal Uupc(11) from mobile station 22 is received by antenna "c" at cell site 11 and detected by the correlation receiver coupled to this antenna at step 600 when the variable i is set equal to channel CH11 and signals are received by antenna "a". If the signal Uupc(11) is higher than the signals received by antenna "a", it is selected as Uup1 at step 601 and the ratio Dupx/Dup1 is compared with K at step 206. As the candidate selection subroutine of Fig. 6 proceeds at base station 11, a signal Uupa(12) on channel CH12 from mobile station 23 is received by antenna "a" and detected at step 600 when variable i is set equal to channel CH12 and signals are received by antenna "c". If the signal Uupa(12) is higher than the signals received by antenna "c", it is selected as Uup2 at step 601 and the ratio Dupx/Uup2 is compared with K at step 206. In the same manner, a signal Uupc(13) on channel CH13 from mobile station 25 is detected by antenna "c" when variable i is set equal to CH13 and compared with signals received by antenna "a" and will be selected as Uup3, and the ratio Dupx/Uup3 is compared with K. If the results of the comparisons successively made at step 206 in the above process indicate that the respective ratios are higher than K, channels CH11, CH12 and CH13 will be stored as candidate channels at step 207. Therefore, if signal Uupc(11) is of highest power level, the ratio Ddn(11 )/Udn(11) will be determined by mobile station 21 to be higher than threshold M at step 213 in the subsequent channel assignment subroutine (see Fig. 2) and channel CH11 will be assigned to mobile station 21. The third embodiment of this invention can be used to advantage if an even number of antennas is provided for all base stations. However, if there exists at least one base station having an odd number of antennas, the third embodiment cannot simply apply. To this end, a three-antenna array is illustrated in Fig. for a base station 11 in the network of Fig. 5 as a fourth embodiment of the present invention. As shown, antennas "a", "b" and "c" have identical directivity patterns Pa, Pb and Pc, respectively, on a horizontal plane and the directivity is represented by relative gains Ga, Gb, Gc each having a fraction of unity relative to their main axis in a manner similar to those shown in Fig. 3. If an undesired signal arrives at an angle β with respect to the zero-degree reference, component signal Uupb will be of highest level and component signal Uupc will be of second highest level. Therefore, the relation Gb(β)/Gc(β) = Uupb/Uupc is established. By using this relation, the angle of arrival β is determined and a correction is made on the highest power level by dividing it with Gb(β) and then multiplying with Gv(β) which is the relative gain at angle β of a virtual directivity pattern Pv. This virtual pattern is obtained by reflecting a copy of the directivity pattern Pa of antenna "a". The relative gain Gb(β), Gc(β) of a pair of adjacent antennas "b" and "c" and the relative gain Gv(β) are stored in a memory, not shown, in which relationships are mapped between arrival angles and relative antenna gains for each pair of adjacent antennas and a virtual directivity pattern resulting from each pair of these antennas. Fig. 8 is a flowchart for implementing the fourth embodiment of the present invention. The flowchart of Fig. 8 is similar to that shown in Fig. 2 with the exception that steps 204 and 205 of Fig. 2 are replaced

Claims: A dynamic channel assignment method for a base station of a mobile communication system, wherein the base station has a plurality of antennas (a, b, c, d; a, b, c) and is capable of selecting one of a plurality of channels of said system for said antennas and corresponding sector areas, the base station being responsive to a call request for a communication with a mobile station for evaluating the qualities of said channels and assigning one of the evaluated channels to the mobile station according to their qualities, the method comprising the steps of: A) determining, for each of said channels, the interference levels of undesired uplink signals (Uupa, Uupb, Uupc, Uupd; Uupx, Uupy; Uupy1, Uupy2) received by all of said antennas; B) determining, for each of said channels, the highest level (Uupi) of said undesired uplink signals; C) selecting one of said channels depending on the highest level (Uupi) of the selected channel relative to the highest levels (Uupi) of other channels; D) assigning the selected channel to the mobile station if the selected channel has acceptable quality; and E) repeating the steps C) and D) if the selected channel has no acceptable quality, wherein the step B) further comprises: B 1 ) determining, for each of said channels, first and second highest interference levels of said undesired uplink signals and selecting first and second antennas (a, b) of said plurality of antennas corresponding to the first and second highest interference levels; B 2 ) determining the arrival angle (α) of said undesired uplink signal of the first highest interference level using directivity patterns of said first and second antennas (a, b); and B 3 ) determining, for each of said channels, said highest interference level using said arrival angle. A dynamic channel assignment method according to claim 1, wherein the step B 2 ) comprises determining a first relative gain (Ga) of the undesired uplink signal of the first highest interference level using the directivity pattern of said first antenna (a) and a second relative gain (Gb) of the undesired uplink signal of the second highest interference level using the directivity pattern of said second antenna (b), said first and second relative gains (Ga, Gb) establishing a proportional relationship with said first and second highest interference levels, and wherein the step B 3 ) comprises determining said highest interference level using the first relative gain (Ga). A dynamic channel assignment method for a base station of a mobile communication system, wherein the base station has a plurality of antennas (a, b, c, d; a, b, c) and is capable of selecting one of a plurality of channels of said system for said antennas and corresponding sector areas, the base station being responsive to a call request for a communication with a mobile station for evaluating the qualities of said channels and assigning one of the evaluated channels to the mobile station according to their qualities, the method comprising the steps of: A) determining, for each of said channels, the interference levels of undesired uplink signals (Uupa, Uupb, Uupc Uupd; Uupx, Uupy; Uupy1, Uupy2) received by all of said antennas; B) determining, for each of said channels, the highest level (Uupi) of said undesired uplink signals; C ) selecting one of said channels depending on the highest level (Uupi) of the selected channel relative to the highest levels (Uupi) of other channels; D) assigning the selected channel to the mobile station if the selected channel has acceptable quality; and E) repeating the steps C) and D) if the selected channel has no acceptable quality, wherein a unique set of channels is allocated to each of said antennas which are oriented in identical directions to the antennas of other base stations, and wherein the step A) comprises determining, for each of said channels, the levels (Uupx, Uupy) of said undesired signal detected by first and second antennas (x, y) which are oppositely oriented to each other. A dynamic channel assignment method for a base station of a mobile communication system, wherein the base station has a plurality of antennas (a, b, c, d; a, b, c) and is capable of selecting one of a plurality of channels of said system for said antennas and corresponding sector areas, the base station being responsive to a call request for a communication with a mobile station for evaluating the qualities of said channels and assigning one of the evaluated channels to the mobile station according to their qualities, the method comprising the steps of: A) determining, for each of said channels, the interference levels of undesired uplink signals (Uupa, Uupb, Uupc Uupd; Uupx, Uupy; Uupy1, Uupy2) received by all of said antennas; B) determining, for each of said channels, the highest level (Uupi) of said undesired uplink signals; C) selecting one of said channels depending on the highest level (Uupi) of the selected channel relative to the highest levels (Uupi) of other channels; D) assigning the selected channel to the mobile station if the selected channel has acceptable quality; and E) repeating the steps C) and D) if the selected channel has no acceptable quality, wherein the step A) comprises determining, for each of said channels, --first and second interference levels (Uupy1, Uupy2) of said undesired signals detected respectively from first and second antennas (y 1 , y 2 ) which form a virtual directivity pattern oriented in a direction opposite to the directivity pattern of a third antenna (x), wherein the step B) comprises: B 1 ) determining the arrival angle (β) of said undesired signal using directivity patterns of the first and second antennas (y 1 , y 2 ); B 2 ) correcting one of the determined interference levels (Uupy1, Uupy2) according to said arrival angle; and B 3 ) selecting the highest one of the corrected interference level and the uncorrected level. A dynamic channel assignment method according to claim 4, wherein the step B 1 ) comprises determining a first relative gain (Gb) of the undesired signal of the first interference level on the directivity pattern of said first antenna (y 1 ), a second relative gain (Gc) of the undesired signal of the second interference level on the directivity pattern of said second antenna (y 2 ), and a third relative gain (Gv) of the undesired signal using said virtual directivity pattern, the first and second relative gains (Gb, Gc) establishing a proportional relationship with the first and second interference levels (Uupy1, Uupy2), and wherein the step B 2 ) comprises correcting the interference level (Uupy1) of the undesired signal detected by the first antenna (y 1 ) using the first and third relative gains (Gb, Gv). A dynamic channel assignment method according to any of claims 1 to 5 wherein the step C) comprises the step C 1 ) of selecting one of a plurality of uplink channels depending on said highest level (Uupi) of the selected channel relative to the highest levels (Uupi) of other channels, and wherein the step D) comprises the step D 1 ) of assigning the selected uplink channel and a corresponding downlink channel to the mobile station. A dynamic channel assignment method according to claim 6, wherein the step D) comprises the steps of: transmitting a carrier of said corresponding downlink channel to the mobile station; and commanding the mobile station to determine the quality of the transmitted carrier and informing the base station of the determined quality.

Inventors: HAMABE KOJIRO, HAMABE, KOJIRO, HAMABE, KOJIRO, C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1994
Main classification: H04Q7
IPC codes: H04W16, H04W72